It has been demonstrated, contrary to conventional wisdom, that laser distance measuring devices can be used as altimeters on board cruise missiles, for example, submunition missiles, dispensers, etc., with an absolutely adequate reliability, since laser beam damping will not be too great, even in poor weather and fog, taking into account the short distances to be measured in this context. Such laser distance measuring devices will be referred to herein as laser distance meters and laser altimeters. Such laser distance meters may also be used when the field of view is directed diagonally or at a slant in the forward and downward direction for the purpose of guiding a terrain-hugging flight with a constant flying altitude above the ground.
In planning a mission, it must always be taken into account that areas over which the missile must fly will be shielded with artificial fog. Visibilities in such artificial fog can sometimes be very short, in extreme cases, less than one meter over larger areas. If an area of terrain is covered with artificial fog, then the upper side or surface of the fog may appear to the laser distance meter, suggested here as a laser altimeter, as a ground surface located closer to the missile. This is so, even though the fog upper surface is only weakly remitting. As a result, the missile goes automatically into a climbing flight. The same effect can also be caused by natural fog when using the am/cw measuring method (amplitude modulation/ continuous wave with phase detection), but in a weaker form.
It is known, that the critical flight phase for cruise missiles, such as dispensers, is the final approach to the target in a terrain-hugging flight at an altitude of approximately 50 m. A laser altimeter suggested for the foregoing purpose has proven itself quite satisfactory, even in heavy, natural fog with a visibility down to about 50 m. In situations wherein the visibility in fog approaches the measurable or measuring distance, laser distance meters functioning according to the am/cw measuring method indicate an increasingly smaller distance than the actual distance with an increasing fog density. As a result, the missile would automatically go into a climbing flight. Therefore, it is necessary that the laser altimeter is capable of recognizing the fog for eliminating the fog's negative influence on the flight guidance. In this connection a method has been suggested which corrects the reduced measured distance with the aid of a known fog extinction coefficient. For obtaining such a coefficient for calibration of the system, measurements are taken of the fog, which is assumed to be constant in its effect throughout its expanse, in a direction from which the fog only, and not a target, reflects the laser beam. German Patent No. 3,606,337 (Sepp et al.), published on Aug. 20, 1987 describes a calibration method for continuous wave laser distance meters based on the just outlined considerations. Corresponding considerations apply to artificial fog, which often has extremely short visibilities, down to less than 1 m and fog layer thicknesses of only a few meters. The heavier the fog is concentrated on the ground, the less falsifying is its influence, with an otherwise constant overall extinction coefficient, on the distance measurement. Yet another method has been suggested for recognizing suddenly occurring artificial fog by reducing the receiving intensity and the distance modulation of the laser altimeter, in response to receiving decreasing distance measured values from the laser distance meter. Such an approach to solving the problem caused by fog can be referred to as a "smoothing" of the ground roughness by the fog, so that the inertial navigation is switched on. However, the above mentioned limitations and assumptions have room for further improvements for avoiding these limitations and assumptions.